Global accountancy firm Deloitte has been hit by a targeted hack, resulting in confidential emails and client plans being compromised.

The attack is thought to have gone unnoticed for several months before the firm being alerted in March this year.

The Guardian reported that the accountancy giant was the victim of a sophisticated hack that revealed confidential information regarding clients, including multinational companies, media enterprises and government agencies.

Six of Deloitte’s clients have been informed that they have been “impacted” by the hack. An internal investigation is ongoing.

Discussing the hack, a spokesperson for Deloitte said: “In response to a cyber incident, Deloitte implemented its comprehensive security protocol and began an intensive and thorough review including mobilising a team of cybersecurity and confidentiality experts inside and outside of Deloitte.

“As part of the review, Deloitte has been in contact with the very few clients impacted and notified governmental authorities and regulators.

“The review has enabled us to understand what information was at risk and what the hacker actually did, and demonstrated that no disruption has occurred to client businesses, to Deloitte’s ability to continue to serve clients, or to consumers.

“We remain deeply committed to ensuring that our cybersecurity defences are best in class, to investing heavily in protecting confidential information and to continually reviewing and enhancing cybersecurity. We will continue to evaluate this matter and take additional steps as required.

“Our review enabled us to determine what the hacker did and what information was at risk as a result. That amount is a very small fraction of the amount that has been suggested.”